1. To avoid a player from accidentally upgrading a tower there should be a physical buttons saying "upgrade" instead of double clicking the tower/ words next to the tower.
2. One thing that is hard to distinct with some enemies is if anti air can kill certain types of enemies. There should be some type of visual queue or label above each enemy saying what type of enemy it is and what can kill it
On another note I feel like to make the game look more visually appealing when defending the city there should be more enemies attacking at a given time. Like ALOT more. Doing thing along with changing the cost of turrents, health, and damage can make the game look at cooler visually.